Aqueous emulsions of polymeric silicone fluids that are stable in terms of particle size are known and used in a variety of applications. Stability is generally achieved by preventing coalescence of the particles by means of ionic or steric repulsion between particles due to the adsorption of a surfactant on the particle surface.
Emulsions of low molecular weight silicones, including cyclosiloxanes such as D.sub.4, D.sub.5, and D.sub.6, and volatile trimethylsiloxy terminated siloxane oligomers such as hexamethyldisiloxane are also known.
Emulsions of these low molecular weight silicone materials can be made mechanically, and some examples of such emulsions can be found, for example, in U.S. Pat. No. 2,755,194 (Jul. 17, 1956); U.S. Pat. No. 4,784,844 (Nov. 15, 1988; and U.S. Pat. No. 5,300,286 (Apr. 5, 1994). However, emulsions of the low molecular weight silicones do not retain their initial particle size, and subsequently increase in diameter with time. This is generally due to a process known as Ostwald Ripening in which a low molecular weight emulsified liquid has a sufficient solubility in water, such that it diffuses from smaller sized particles or droplets through the aqueous phase into larger particles. This results in a net increase in average particle size of the emulsion with time.
This phenomenon leads to an inconsistent behavior or performance pattern of the emulsion in commercial use, due to the changing properties of these emulsions with age.
This invention, in contrast, is intended to provide an effective avenue for preventing, or at least minimizing, the increases in emulsion particle or droplet size with age; with the result that it provides for production of stable emulsions of low molecular weight silicone oils. For purposes of this invention, the phrase low molecular weight is intended to mean a molecular weight generally less than about 1,000. Thus, it is considered that silicones having a molecular weight in excess of about 1,000 have no significant solubility in water, and hence would not be adversely effected by Ostwald's Ripening phenomenon.
This invention, it is noted, further differentiates from what is known in the prior art, in that, with respect to the '194 patent, the HLB of the nonionic cosurfactant according to this invention is less than 11, compared to the HLB of the nonionic emulsifier in the '194 patent which is 14.1. With respect to the '844 patent, the present invention employs a total in parts by weight of emulsifiers per 100 parts by weight of silicone of only 1.4-218, compared to the use of 0.7-666 parts by weight of emulsifiers per 100 parts by weight of silicone in the '844 patent. Thus, the range in this invention is within the range of the '884 patent, but it is narrowly focused and is accompanied by new and unexpected results, i.e., the emulsions do not increase in particle size with aging. Regarding the '286 patent, while Table XI in the '286 patent refers to the use of a single silicone, it has a viscosity of 1,000 centistoke, whereas the single silicone used in this invention has a viscosity of no greater than 5 centistoke.